Use of liquefied gas as a fuel source for various applications has gained popularity in recent years due to the lower cost and cleaner burning of gaseous fuels such as liquefied petroleum gas (LPG), compressed natural gas (CNG), or liquefied natural gas (LNG), as compared to more traditional fuels such as gasoline or diesel. In practical applications, for example, mining trucks, locomotives, highway trucks and the like, the engine may operate primarily on natural gas, which is a fuel requiring ignition. Ignition may be provided by a spark or by introduction of a compression ignition fuel such as diesel at pilot quantities within the engine cylinders.
Typical compression ignition engines such as diesel engines operate under relatively high compression ratios, for example, in the range of 16:1 to 17:1, which are required for diesel self ignition within the engine cylinders during engine operation. Spark ignition engines such as engines operating under an Otto cycle utilize a spark or glow plug to ignite the air/fuel mixture within the engine cylinders at a predetermined time. In spark ignition engines, self-ignition of the air/fuel mixture is undesired and, typically, detrimental to normal engine operation. Spark ignition engines typically use lower compression ratios than compression ignition engines. For example, natural gas engines may use a compression ratio of about 11:1 to 12:1.
As can be appreciated, compression ignition dual fuel engines using a diesel pilot to ignite a natural gas/air mixture are faced with conflicting design parameters because, although a high compression ratio is required to ignite the diesel pilot, the high compression ratio may also cause the natural gas to prematurely self-ignite. When low compression ratios are used to avoid self-detonation of the natural gas/air mixture, insufficient cylinder temperatures and pressures may lead to unacceptable variability in the diesel ignition, which can affect normal engine operation especially when lean air/fuel ratios are used.